1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly although not exclusively, to a zoom lens system which can be used with or in an optical photography system.
2. Description of the Related Art
In recent years, with improvements in video cameras and digital still cameras using solid-state image pickup elements, there has been a demand for an image pickup optical system having a zoom lens that include a wide field angle and a large aperture ratio which can be used in the cameras.
In this type of camera, various types of optical members (e.g., a low pass filter and a color correction filter) can be disposed between a lens backmost portion and the image pickup element. Therefore, a lens system having a comparatively back focus is required for the optical system to be used in conventional cameras. Furthermore, in the camera using the image pickup element for a color image, the optical system which can be used in the camera is required to have a satisfactory telecentric property on an image side in order to avoid color shading.
Heretofore, conventional systems have proposed various two-unit zoom lenses of a so-called short zoom type which is constituted of two lens units: a first lens unit having a negative refractive power; and a second lens unit having a positive refractive power and which performs zooming while changing an interval between lenses. In these short zoom type of zoom lenses, the second lens unit which can have the positive refractive power is moved to vary a magnification, and the first lens unit which can have the negative refractive power is moved to compensate for an image position in association with the varying of the magnification. In such two lens units, a zoom ratio is about two.
Furthermore, to form the whole lens into a compact shape while achieving a two or higher zoom ratio, conventional systems proposed a so-called three-unit zoom lens in which a third lens unit having a negative or positive refractive power is disposed on an image side of the two-unit zoom lens (e.g., Japanese Examined Patent Application Publication (Kokoku) No. 7-3507 (corresponding to U.S. Pat. No. 4,810,072) and Japanese Examined Patent Application Publication (Kokoku) No. 6-40170 (corresponding to U.S. Pat. No. 4,647,160)). Conventional system have discussed various configurations of three unit zoom lens (e.g., Japanese Unexamined Patent Publication (Kokai) No. 63-135913 (corresponding to: U.S. Pat. No. 4,838,666), Japanese Unexamined Patent Publication (Kokai) No. 7-261083).
In a conventional three-unit zoom lens discussed in Japanese Unexamined Patent Publication (Kokai) No. 3-288113 (corresponding to U.S. Pat. No. 5,270,863), a zoom lens includes a fixed first lens unit having a negative refractive power, and a second and third lens units having a positive refractive powers which are moved to perform zooming.
Moreover, in the three-unit zoom lens where the first lens unit has a negative refractive power, and the second and third lens units have a positive refractive power, conventional systems discuss a configuration where the second lens unit is constituted of a positive first lens subunit, a positive second lens subunit, a negative third lens subunit, and a positive fourth lens subunit (e.g., Japanese Unexamined Patent Publication (Kokai) No. 9-258103 (corresponding to U.S. Pat. No. 5,872,660), Japanese Unexamined Patent Publication (Kokai) No. 11-52246 (corresponding to U.S. Pat. No. 6,124,984), Japanese Unexamined Patent Publication (Kokai) No. 11-174322, Japanese Unexamined Patent Publication (Kokai) No. 11-194274, Japanese Patent No. 3466385, Japanese Unexamined Patent Publication (Kokai) No. 2002-23053 (corresponding to U.S. Pat. No. 6,618,210), Japanese Unexamined Patent Publication (Kokai) No. 2002-196240 (corresponding to USAA 2002149857)).
Other conventional systems discuss three-unit zoom lens, which are constituted of lens units having a negative, positive, and positive refractive powers, with a three or higher zoom ratio (e.g., Japanese Unexamined Patent Publication (Kokai) No. 4-217219, Japanese Unexamined Patent Publication (Kokai) No. 10-039214, Japanese Unexamined Patent Publication (Kokai) No. 10-213745, Japanese Unexamined Patent Publication (Kokai) No. 11-119101 (corresponding to U.S. Pat. No. 6,038,084), Japanese Unexamined Patent Publication (Kokai) No. 11-174322, Japanese Unexamined Patent Publication (Kokai) No. 2001-42218 (corresponding to U.S. Pat. No. 6,304,389, Japanese Unexamined Patent Publication (Kokai) No. 2002-365545 (corresponding to USAA 2003103157), Japanese Unexamined Patent Publication (Kokai) No. 2002-267930 (corresponding to U.S. Pat. No. 6,498,688), Japanese Unexamined Patent Publication (Kokai) No. 2003-156686, and Japanese Patent No. 2895843 (corresponding to U.S. Pat. No. 4,828,372.)).
The conventional three-unit zoom lens designed for a 35 mm film photograph has such an excessively long back focus and unsatisfactory telecentric property that the lens is typically not suitable which can be used in image pickup apparatus using the solid-state image pickup element.
On the other hand, a so-called retracting type of zoom lens has been broadly used in which at a non-photographing time, an interval between the lens units is reduced to an interval that is different from that at a photographing time, and a length of the lens protruded from a camera main body is reduced in order to establish both a compact size of the camera and a high zoom ratio of the zoom lens.
In general, when the number of the lenses of each lens unit constituting the zoom lens is large, the length of each lens unit on an optical axis increases. When a movement amount of each lens unit in zooming and focusing is large, a total length of the lens system increases. As a result, typically a desired retracted length cannot be achieved, and it becomes difficult to use a retracting type of zoom lens. This tendency increases, when the zoom ratio of the zoom lens increases.
On the other hand, when a non-spherical lens is used, the number of the lenses can be reduced. However, the non-spherical lens is more expensive than a spherical lens. Therefore, when the number of the non-spherical lenses is increased, costs will increase.